The present invention relates to a method for aligning first and second objects relative to each other, and, more particularly, to a method for aligning a mask and a wafer when a circuit pattern is to be transferred onto a wafer, in the process of manufacturing a semiconductor device. Furthermore, the present invention relates to an apparatus for aligning first and second objects and, more specifically, to an apparatus for aligning a mask and a wafer.
In the process of manufacturing a semiconductor device such as an LSI, an optical projection/exposure apparatus is used for transferring a circuit pattern onto a wafer. Using this apparatus, a circuit pattern pre-formed on a mask is reduced in scale and then transferred onto a wafer by a optical projection system and a mercury lamp. Before the circuit pattern is transferred, however, the mask and the wafer must be precisely aligned (mask alignment). More specifically, the mask and the wafer must be aligned in a direction perpendicular to the direction of light projection onto the circuit pattern.
As examples of mask alignment methods, the TTL (through-the-lens) method and the off-axis method are known. When using the TTL method, a light beam from a projection optical system is projected onto diffraction gratings formed on each of a mask and a wafer, in order to detect the relative position of the mask and the wafer. In contrast, when using the off-axis method, an off-axis microscope, instead of the projection optical system, is used to align the mask with the wafer. The mask and the wafer are individually aligned with the off-axis microscope, outside the projection range of the circuit pattern. Then, the mask and the wafer, which are aligned with each other, are moved to within the projection range. The time for positioning, needed for the off-axis method is shorter than that needed in the TTL method. However, the off-axis method involves many error factors, and high-precision alignment is hard to attain. For this reason, the TTL method is the most widely-used method for aligning the mask and the wafer.
One TTL mask alignment method, which involves completely overlapping two grating patterns, is disclosed in the article, G, Dubroeucq, 1980, ME, W. R. Trutna, Jr., 1984 SPI. When this method is employed, laser beams are radiated onto grating patterns respectively formed on a mask and a wafer, and diffracted beams are incident on a photodetector, so as to be converted to an electrical signal. The electrical signal is processed by a processor, and the relative position of the mask and the wafer is detected. The mask and the wafer are aligned based on the detection result.
As is described above, when using the TTL method, laser beams are radiated through a projection lens of the projection optical system. The projection lens is arranged to minimize all aberrations with respect to a predetermined exposure wavelength of a mercury lamp. For this reason, when light which has a wavelength other than the exposure wavelength is radiated through the projection lens, chromatic aberration appears. The relative position of the mask and the wafer is detected, in a state wherein the distance between the mask and the wafer corresponds to a defocused state. In this case, sufficient detection sensitivity cannot be obtained, and reproducibility is also poor. Therefore, an error may occur in the alignment of the mask and the wafer. When the difference between two wavelengths becomes large, an aberration-correction means becomes necessary, in order to correct the optical path length.